1. Field of the Invention
The present invention relates to an improved process for selectively hydrogenating acetylenes and dienes in a hydrocarbon stream in a downflow boiling point reactor.
2. Related Information
The vapor product stream from the quench system of a hydrocarbon steam cracker typically consists mainly of hydrogen, methane, C2-C6 olefins and paraffins, C2-C6 acetylenes and dienes, benzene, toluene, xylenes, and other C6+ components. Separation and recovery of the products according to carbon number is generally accomplished in a sequential distillation system after the first separation of hydrogen from the methane in a high pressure cold box system. The design of the distillation system is complicated by the fact that the differences in relative volatility of the olefins, acetylenes, and dienes of the same carbon number are small making it difficult to produce the pure olefin products. One method of circumventing this problem is to first separate the carbon number fractions and then to selectively hydrotreat each fraction to convert the acetylene and/or diene to its corresponding olefin or paraffin. This so called “back end” approach requires a separate hydrotreating system for each carbon number fraction as well as the addition of a requisite amount of hydrogen to each system. An alternative method is to hydrotreat the feed stream before separation using the contained hydrogen as the source of hydrogen for the conversion. This so-called “front end” approach has the advantage of removing a significant portion of the hydrogen from the feed stream to the cold box thereby reducing the size and refrigeration requirements of the cold box.
U.S. Patent Publication 2003-0233017-A1 (Dec. 18, 2003) discloses a method to maintain mixed phase flow and temperature control through the downflow boiling point reactor by removing a relative large liquid stream enriched in C6+ components from a C5/C6 splitter and feeding it to the reactor together with the net vapor feed and reactor liquid recycle stream whereby the C6+ components are returned to the splitter in the vapor fraction from the reactor vapor/liquid separation drum.
It is an advantage of the present invention that the withdrawal and return of the C6+ components from the C5/C6 splitter is eliminated. It is a further advantage that a smaller C5/C6 splitter and associated reboilers and condensers are required.